This invention relates to apparatus for the conversion of solid organic materials, having a fuel value, by high temperature gasification into a clean-burning and uniform gaseous fuel called "producer gas." More particularly, the invention relates to a gasifier in which peat, lignite, coal, anthracite, coke and other solid organic materials (including solid organic waste materials such as wood chips, sawdust, pine cones, other forest wastes, walnut and almond shells, fruit pits, corn cobs and other agricultural wastes, and like materials) used as solid fuels are transformed into gaseous fuels and in the process of such transformation carry the largest possible amount of energy from the solid to the gaseous state while keeping the resulting gaseous fuels clean and free from undesireable constituents of the solid fuels.
Solid fuels of the type mentioned above, in addition to containing the impurities of ash and water, consist of so-called fixed carbon and volatile matter. The objective of gasification is to obtain substantially complete transformation of the gasifiable constituents of the solid fuel into fuel gases leaving only ash and inert materials as a solid residue of the conversion process. Gasification occurs when air is led across or drawn through glowing hot gasification material, the stream of air being either dry or containing steam. Usually gasification occurs under the influence of steam since the air flow always contains some moisture and the solid fuels normally utilized in gasification systems contain moisture. Further, such solid fuels usually contain some hydrogen which reacts with oxygen creating steam.
Gasification systems and their associated gasifier apparatus have generally fallen into one of three classifications as follows: (1) updraft gasification; (2) downdraft gasification; and (3) crossdraft gasification. Under each classification a column of the solid fuel to be gasified is developed in a reactor or stack and air is passed through the column. As the fuel gasification proceeds the column gradually moves downwardly within the reactor or stack into a lower hearth zone. The air stream can be led in the same direction as the direction of fuel movement (downdraft gasification) or led in a direction opposite to the direction of movement of the descending fuel column (updraft gasification). If the air stream traverses the descending fuel column crossdraft gasification is promoted. Each method allows the fuel to gradually enter the hearth zone where highest temperature conditions subsist.
In the basic form of an updraft gasification system the fuel column rests on a grate through which a stream of air and steam passes. Above the grate a hearth zone develops with a reduction zone, a distillation zone and a drying zone lying sequentially above the hearth zone within the fuel column. The product gas is drawn off above the fuel column after having transferred some of its heat to the fuel in the distillation and drying zones in the upper part of the column. Only tar free fuels such as charcoal or anthracite are suitable for updraft gasification systems. If the fuel contains tar, as do wood, peat, lignite, etc., the tar is gasified and carried off with the producer gas generated through the gasification system. A tar separator is then required to prevent the tars from fouling or otherwise adversely affecting downstream equipment.
In downdraft gasifiers the air stream enters the system in the area of the hearth zone (usually through nozzles arranged circumferentially or through a central nozzle) and draws all of the gaseous fuel components down into the hearth zone, there to enter into the gasification reactions. Tars and moisture are exposed at high temperature to the carbon in the hearth zone and undergo partial combustion and partial dissociation so that the final producer gas leaving the system is tar free. Downdraft gasification systems have developed a characteristic funnel shaped constriction of the hearth at or just below the entry of the air stream. The hearth constriction or throat causes a localized increase in the air flow velocity which in turn causes localized high temperature conditions for conversion of the tars into their gaseous components. Downdraft gasifier operation is generally unsuitable for fuels with high ash content because the high temperatures generated in the throat section of the hearth cause sintering of the ash into a slag which is difficult to remove and causes functional problems in the system.
In crossdraft gasification air is introduced through a small diameter high velocity nozzle and is projected across the fuel column to achieve a hearth zone of small volume but of very high temperature. Tar dissociation is limited because of the small hearth zone that is developed and therefore low tar fuels are preferred for crossdraft gasification.